The exemplary embodiment relates to the illumination arts. It finds particular application in connection with a lamp for use in a night vision system and will be described with reference thereto.
Vehicle headlights and fog lights provide visible illumination which assists the driver in seeing the road ahead. However, because the beams are angled downwards to prevent interference with the vision of oncoming drivers, they tend not to provide optimal illumination of potential road hazards. The driver has a relatively short time to react to the presence of hazards, such as stalled vehicles, road construction, wild animals, road debris, and the like. Various night vision systems have been developed to assist drivers. During night and adverse weather, night vision systems provide drivers with supplemental visual information, beyond the range of their headlamps. The large range provides the driver with more time to react in situations that pose unexpected danger. Such systems employ a camera which detects infrared radiation. The imaged radiation is visualized as a representational image which is presented to the driver via a video monitor mounted in the passenger compartment or through a head-up display, which projects the image congruently with the outside world into the driver's eye. These systems allow maximum illumination of the viewing region and therefore provide the driver with a good view when it is dark but without blinding other road users.
Two types of systems have been developed. Passive infrared systems detect objects based on their emitted thermal radiation in the far infrared (FIR). Only relatively warm objects, such as people and animals are detected, they do not readily detect objects which emit radiation at or close to background levels, such as stalled vehicles and road debris. Additionally, the detection systems employed to collect and analyze the infrared radiation are relatively complex and not generally amenable to passenger vehicles. Active, near infrared (NIR) systems use an IR-source to project infrared radiation onto a scene and image the radiation that is reflected by objects in the scene. This provides a relatively complete picture of the scene in front of the driver. Effective infrared sources are halogen lamps which emit a sizable portion of their radiation in the near infrared. However, they also emit in the visible range, which in automotive applications could blind an oncoming driver. A filter which filters out the visible light may be positioned in front of the lamp. Such filters, however, can become warped or damaged in use, allowing visible light to penetrate.
EP1072841A2, for example, discloses an infrared headlight in which an incandescent lamp is used as the infrared radiation source, whose incandescent filament emits both infrared radiation and light in the visible range during operation. A parabolic reflector deflects the infrared radiation to the desired direction and transmits the visible radiation. The reflector opening is covered by a filter disk, which is opaque to light in the visible range. Planar filters such as these are not generally suitable for automotive applications because the filtered radiation tends to produce a large amount of heat which can damage the filter.
DE3932216A1 discloses an illumination device for automotive applications, which can be used both as an infrared headlight and as a main beam. The illumination device has a reflector in which a light source is positioned. Infrared radiation can pass through a filter which reflects radiation in the visible range towards the light source. In the main beam mode, the filter is moved with respect to the light source such that it is ineffective, so that all of the radiation is reflected via the reflector towards the reflector opening. When dipped lights are selected, the filter is moved over the light source such that the illumination device emits only infrared radiation. This limits the device to the shorter distances which can be reached with the dipped beam.